1. Field of Invention
The present invention relates to an illumination light source with little speckle noise and a laser projection device using such an illumination light source.
2. Description of the Related Art
Semiconductor lasers made of III-V nitride semiconductor materials (AlxGayIn1-x-yN (where 0≦x≦1, 0≦y≦1), such as gallium nitride and red semiconductor lasers made of AlGaAs semiconductor materials or AlGaInP semiconductor materials are key devices for achieving ultra-high density recording by optical disks. Similarly, higher outputs of these visible light semiconductor lasers not only enable high-speed writing of optical disks, but also are essential technology for pioneering new technological fields such as application to laser displays.
What is problematic in the case of utilizing a visible light semiconductor laser as an illumination light source for a projection device, a display device or the like is speckle noise. Speckle noise is a phenomenon in which a light reflected from an object being illuminated has its wavefront disturbed by the unevenness of the surface of the object being illuminated and a random interference pattern is observed in the case where a light with high coherence such as a laser light is utilized as an illumination light source. A glaring speckle pattern is observed in the reflected light, which causes the image degradation of the projection device and the display device.
Several methods for reducing the coherence of a light source have been proposed as methods for reducing such speckle noise. The first method is for reducing spatial coherence and represented by the vibration of a screen, to which a laser light is projected. According to this method, speckle noise is reduced by giving a spatial variation in an optical path of the laser light and an optical system.
On the other hand, there is also proposed a method for directly reducing the coherence of a semiconductor laser light. This is a method for reducing the coherence of a light source as disclosed in patent literature 1. According to this second method, the spectral width of an oscillation wavelength is increased to reduce the coherence by superimposing a high frequency wave on the drive of the semiconductor laser.
However, the above first method for spatially changing the laser light requires a mechanical driving system in the optical system and has a problem of enlarging and complicating the optical system. Further, it is difficult to completely suppress the speckle noise only by giving the spatial variation.
On the other hand, the above second method for multimoding the spectrum of the semiconductor laser is effective in reducing the coherence, but no sufficient coherence reducing effect can be obtained unless the spectral width is increased to 1 nm or larger. Only by superimposing the high frequency wave on the drive of the semiconductor laser, there has been a problem that an increase of the spectral width and a speckle noise reducing effect are insufficient.
Patent Literature 1
Japanese Unexamined Patent Publication No. 2002-323675